A multimillion-dollar airflow study will be conducted this summer to help authorities better understand the risks of terrorist attacks using toxic airborne materials on New York City and its subway system, including those posed by chemical, biological and radiological weapons.

The NYPD and experts from the U.S. Department of Energy's Brookhaven National Laboratory will conduct the tests in July. The Subway-Surface Air Flow Exchange study, or S-SAFE, is funded by a $3.4 million transit security grant from the U.S. Department of Homeland Security.

But the NYPD statement Wednesday characterized per-fluorocarbon tracer gases as "harmless." It described them as nontoxic, inert, odorless and invisible.

The research is scheduled for daylight hours at locations in the Bronx, Brooklyn, Queens and Staten Island, as well as in Manhattan between 59th Street and the Battery, officials said.

The NYPD said it will use the data collected from the study to "optimize emergency response" actions if there were an intentional or accidental release of hazardous materials into the atmosphere.

The main focus, however, remains to formulate a response to a potential terrorist attack.

"The NYPD works for the best but plans for the worst when it comes to potentially catastrophic attacks such as ones employing radiological contaminants or weaponized anthrax," NYPD Commissioner Ray Kelly said in a statement Wednesday, adding: "This field study with Brookhaven's outstanding expertise will help prepare and safeguard the city's population in the event of an actual attack."

The study will help the NYPD and other emergency responders to better understand how contaminants disperse in the atmosphere -- and what can be done to better protect the population both above and below ground in an attack.